This invention relates to producing polymer of increased molecular weight by reacting a lithium-initiated polymer with a coupling agent.
Many polymers of conjugated dienes synthesized employing organolithium initiators have a strong tendency to cold flow in the uncured state. For example, this tendency commonly occurs with polymers when their Mooney value is below 30 (ML-4 at 212.degree. F.). A very attractive solution to this problem has been to couple two or more polymer molecules by reaction of the "living" or metal terminated polymer with a compound containing two or more sites capable of reacting with the carbon-lithium bond and adding to the carbon atom possessing this bond in the polymer. The resulting coupled polymer exhibits little or no tendency to cold flow, a very marked increase in Mooney, good processability and the absence of gel. Further, coupling of certain appropriate polymerlithium types can produce thermoplastic elastomers. For example, dichain coupling of block copolymers of type AB-Li wherein A is a block of monovinylarene and B is a block of acyclic conjugated diene yields a thermoplastic elastomer of structure ABBA or, broadly, ABA.
However, coupled polymers differ in their stability. As an example, it has been found that some coupled polymers tend to suffer cleavage, at least in part, back to the parent or uncoupled polymer when said coupled polymers are subjected to catalytic hydrogenation. This tendency can be a serious limitation when it is desired to hydrogenate a polymer to improve its environmental stability, e.g., its stability to ozone. However, this tendency is not exhibited by coupled polymers that became coupled through the formation of carbon-carbon bonds.